The present invention relates to thin film ternary chalcopyrite semiconductors and in particular to a combined chemical vapor deposition/thermal ion exchange process for producing photoconducting films of zinc tin phosphide.
The subject invention was made in the course of research performed for the Solar Energy Research Institute under contract XS-9-8041-11.
Zinc tin phosphide (hereinafter ZnSnP.sub.2) has heretofore been prepared from solution growth techniques in which constituent elements were combined at their melting points to form the ternary compound. The specific example of this technique may be referenced in a technical publication entitled "Preparation and Characteristics of Zinc Tin Phosphide" by M. Rubenstein et al published in the Journal of Physics and Chemistry of Solids, Volume 29, pages 551-552 (1968). The solution growth technique typically results in very small crystallites having their largest dimension on the order of millimeters. In contrast the present invention is directed to a technique for producing thin films of zinc tin phosphide which may be used in large area applications. Process for producing these films includes both a chemical vapor deposition and a thermally induced ion exchange.
An alternative technique for the preparation of ternary zinc tin phosphide is taught by S. A. Mughal et al in a technical publication entitled "Preparation and Phase Studies of the Ternary Semiconductor Compounds Zinc Tin Phosphide, Zinc Germanium Phosphide, Zinc Silicon Phosphide, Cadmium Germanium Phosphide and Cadmium Silicon Phosphide" published in the Journal of Materials Science, Volume 4, Pages 895-901 (1969). Mughal et al teaches the use of a carrier gas comprising iodine or chlorine to transport the constituents of the ternary compound. As taught therein, the ternary chalcopyrite semiconductor is prepared by heating its elemental constituents in a sealed ampule containing an amount of the carrier gas. A cooled region of the ampule serves to nucleate crystallites of the desired compound. In contrast the present invention teaches the preparation of thin films of photoconductive zinc tin phosphide from a process that is believed to comprise the chemical vapor conversion of tin oxide to a zinc tin oxide and concurrently ion exchanging the oxygen for phosphorous to form the desired film of zinc tin phosphide.